Erase is possible in watercolor, within the first few seconds and when it is wet enough. I know I should not encourage this "trick", because I am a big believer in focus on doing it right without fixing it later. But I cannot deny there are times I took the wet paint out so I can give myself a second chance to make it right. To "erase" the error, you have to do it when the paint is still wet. Press it with a piece of dry paper towel and blot it out. If you do it fast enough, the paper towel will absorb the paint out before it settles on the paper. This trick usually works better with very wet mixtures. If the paint is dry, you are less likely to be able to get it out. It is also worth noting that you will still left with a light stain on the paper, but in most cases it is negligible and will disappear when you put another wash over it. Dryer mixture can also be taken away by adding clean water and blot it out with paper towel. The stain might be a bit more heavier, but it will still be better than leave a bad mark and let it dry.

Just because you know this trick, doesn't mean you can do it in time when it's called for.

Even though I use this trick more often than I'm willing to admit, the truth is you actually need to be able to spot the error fast enough in order to make use of this trick. Most of the time I already had a good idea what the wash should look like before I put the paint down. If you don't know beforehand how your wash should look like, you won't be able to fix it before it's too late. There are students came to me and ask me how to fix it after the paint is long dried, and the sad truth is there is no way to fix it. So just because you can do it, doesn't mean you don't need to learn how to paint well. After all, this is watercolor, not Photoshop, you do not get a Ctrl-Z.